The present invention relates to a vector control apparatus for an induction motor and, more particularly, to a vector control apparatus for an induction motor wherein rotational smoothness of the motor at a low rotation speed with a small torque can be greatly improved and a loss of the motor with a large torque can be greatly reduced.
Conventional induction motors have been used as constant speed motors using a power source of a predetermined frequency in a variety of applications due to rigidness and low cost.
Along with the recent development of electronic devices, microcomputers, and software, a power source having a wide variable frequency range can be obtained to drive an induction motor. The field of applications of the induction motors is changing from constant speed motors to servo motors. The variable frequency power source is operated according to vector control.
Basic parameters in vector control are a torque current i.sub.1q, an exciting current i.sub.0 for generating a secondary flux .PHI..sub.2, and a slip speed .omega..sub.s and are defined as follows: EQU i.sub.1q =(L.sub.22 /M) (T/.PHI..sub.2) (1) EQU i.sub.0 ={(.PHI..sub.2 +(L.sub.22 /R.sub.2).multidot.(d.PHI..sub.2 /dt)}/M(2) EQU .omega..sub.s =TR.sub.2 /.PHI..sub.2.sup.2 =(R.sub.2 /L.sub.22).multidot.(i.sub.1q /i.sub.0) (3) EQU for the steady state, i.e., d.PHI..sub.2 /dt=0
where L.sub.22 is the inductance of the secondary winding, M is the mutual inductance between the primary and secondary windings, T is the torque, .PHI..sub.2 is the flux generated by the secondary winding and crossing with the primary winding, and R.sub.2 is the resistance of the secondary winding. Relational equations (1), (2), and (3) are referred to as vector relational equations.
The secondary flux .PHI..sub.2 is a predetermined value in vector control The torque T is a command value supplied to the vector controller for d.PHI..sub.2 /dt=0 as follows: EQU T=(M.sup.2 /R.sub.2).omega..sub.s i.sub.0.sup.2 =(M.sup.2 /L.sub.22)i.sub.0 i.sub.1q . . . (4)
That is, a so-called inverter is controlled by the slip speed .omega..sub.s, the exciting current i.sub.0, and the torque current i.sub.1q to supply power to the induction motor so as to allow the motor to operate at desired ratings.
FIG. 1 is a block diagram of a conventional vector control system shown in "New Drive Electronics", Naohiko Kamiyama, P. 205. In other words, FIG. 1 shows a basic arrangement of conventional "slip frequency type vector control".
Referring to FIG. 1, reference numeral 1 denotes a speed control amplifier for generating a torque T; 2, a divider; 3, a constant setter for outputting the torque current i.sub.1q ; 4, a vector analyzer; 5, a multiplier; 6, a converter; 7, a current amplifier; 8, a power converter; 9, an induction motor; 11, a speed detector; 12, a differentiator; 13, 14, 15, and 16, constant setters for generating the exciting current i.sub.0 ; 17, a divider for generating the slip speed .omega..sub.s ; 18, a vector oscillator; and 20, an adder. With the above arrangement, the torque can be controlled in accordance with an instantaneous current. Please refer to PP. 205-206 in the above reference for the operation of the circuit shown in FIG. 1.
In the case of the slip frequency type vector control circuit shown in FIG. 1, an expected value .PHI..sub.E of the secondary flux .PHI..sub.2 is generally constant within the entire rotation speed range and the entire torque range (this is referred to as constant torque characteristics, so that an output from the induction motor is increased in proportion to the motor rotation speed), as shown in FIG. 2.
If a constant output of the induction motor in a high-speed range is required, the secondary flux .PHI..sub.2 is kept constant at a rotation speed below a predetermined rotation speed .omega..sub.r11, as shown in FIG. 3. However, at a rotation speed exceeding the predetermined rotation speed .omega..sub.r11, the secondary flux .PHI..sub.2 is in inverse proportion to the rotation speed .omega..sub.r (i.e., constant output characteristics). In this case, the secondary flux .PHI..sub.2 is a function of the rotation speed .omega..sub.r.
However, if the induction motor is used as a servo motor, the following problems are presented. The servo motor must satisfy the following requirements: (1) smooth rotation, i.e., a small variation in rotation speed is required in mainly a low-speed range in order to achieve high-precision control, for example, in table feed finish machining in a machine tool; and (2) a heat loss must be minimized and a torque must be maximized in a high-output operation in, e.g., table feed coarse machining in the machine tool.
In conventional slip frequency type vector control, the requirements of the servo motor cannot be sufficiently satisfied. More specifically, the loss of the output in a high torque operation cannot be reduced and smooth rotation at a low speed with a small load cannot be achieved.